Removal of material from driving area

Category: MODIFYING THE SLOPE GEOMETRY - mass distribution

Description

The removal of material from the driving area (or more in general, regrading or flattening slope angle) operates by reducing the driving forces, thereby improving overall slope stability.

This method is most suitable in cases where the instability mechanism occurs as a rotational or pseudo-rotational slide, e.g. where the displaced mass moves as a relatively coherent mass along a spoon-shaped (curved upward) failure surface with little internal deformation. It is generally ineffective on translational slides on long, uniform planar slopes, or on flow-type landslides.

Generally it is most practical on small slumps or small rotational failures, but several examples exist where this technique has been applied successfully on large landslides where conditions allowed large scale earthmoving to be carried out.

It should always be kept in mind that the resisting forces are also reduced, especially in the long term, as a result of the reduction in normal stress on the failure surface. It is therefore necessary to locate the excavation in such manner that the reduction in driving forces exceeds the reduction in resisting forces. The neutral line concept, described in fact sheet 2.0 on “mitigation by modifying the slope geometry / mass distribution; general aspects” can be used for a preliminary evaluation of the relative merits of the proposed excavation.

The main limitations of the technique relate to the following issues:

  • Excavation may actually destabilize the ground farther up-slope by ubdercutting ;

  • Excavation increases safety factor by only a limited amount, which tends to decrease with time in low permeability saturated soils; satisfactory solutions may involve significant modification of the landscape (see for example Chatwin et al.,1994);

  • Excavation results in large volumes of material to be disposed of off-site in a controlled manner, with attendant difficulties;

  • Excavation may interfere with existing structures and services; This is potentially significant when considering this type of mitigation for “potential” landslides, while on actual landslides the residual value of existing structures and facilities can be very low;

  • Excavation impacts on the upper part of the slope, with the greatest potential visual impact on the landscape

  • Excavation of active landslides requires special care to ensure the safety of workers; in particular, it is necessary to assess the possibility of sudden accelerations and to have in place well drilled evacuation plans.

All excavation in the upper part of a landslide must be accompanied by drainage works to redirect surface water away from infiltrating the landslide body. Typically, surface protection to newly excavated surfaces is also necessary to limit erosion and/or weathering. To facilitate construction and maintenance of drainage and surgface protection works, excavated surfaces are typically shaped to form a number of benches, typically at 6 to 10 m vertical interval.

Examples of large landslides stabilized by this technique are shown in Figures 1 and 2.

Figuere 1: Cameo Slide, Colorado River Valley – Stabilization by partial removal of material in driving area (Volume B). Removal of volume A was considered and found ineffective (source Peck and Ireland, 1953; Baker and Marshall, 1958)
Figuer 1: Cameo Slide, Colorado River Valley – Stabilization by partial removal of material in driving area (Volume B). Removal of volume A was considered and found ineffective (source Peck and Ireland, 1953; Baker and Marshall, 1958)

 

Figure 2: Cortes de Pallas Landslide, Spain – Stabilization by excavation in driving area (source Alonso et al., 1993)
Figure 2: Cortes de Pallas Landslide, Spain – Stabilization by excavation in driving area (source Alonso et al., 1993)

 

Figures 3 and 4 show the remedial works carried out at the Settebagni motorway cutting, just North of Rome, where major deep seated sliding occurred approximately 20 - 25 years after construction due to a thick plio-pleistocene clay layer daylighting in the cutting below a thick cover of otherwise stable tuffs and pyroclastic cinders (Figures 5 and 6). The extent of the clay outcrop in the cutting is shown indicatively by the hard facing installed at the time of construction to safeguard fron erosion and shallow instability. As shown in the figures, reprofiling formed an essential part of stabilization works and extended for the full portion of the cutting potentially affected by future sliding, beyond the limits of the 1992 slide (SGI-MI project files).

 

Figure 3: Remedial works for Settebagni slide included major reprofiling from the original 1960’s cut slope profile to reduce driving forces (SGI-MI project files)
Figure 3: Remedial works for Settebagni slide included major reprofiling from the original 1960’s cut slope profile to reduce driving forces (SGI-MI project files)

 

Figure 4: Remedial works for Settebagni slide included major reprofiling from the original 1960’s cut slope profile to reduce driving forces; note original grading in southern portion of cut (SGI-MI project files)
Figure 4: Remedial works for Settebagni slide included major reprofiling from the original 1960’s cut slope profile to reduce driving forces; note original grading in southern portion of cut (SGI-MI project files)

 

Figure 5: Settebagni slide – 1992 after emergency temporary remedial works (SGI-MI project files)
Figure 5: Settebagni slide – 1992 after emergency temporary remedial works (SGI-MI project files)

 

Figure 6: Settebagni slide – 1992 after emergency temporary remedial works (SGI-MI project files)
Figure 6: Settebagni slide – 1992 after emergency temporary remedial works (SGI-MI project files)

 



Design methods

For general considerations on the geotechnical design of mitigation by removal of material from the driving are, reference shall be made to the general fact sheet 2.0 on hazard mitigation by changes in slope geometry and/or mass distribution.



Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 2 Will be updated soon
Topple 3
Slide 8
Spread 2
Flow 2

Material type

Descriptor Rating Notes
Earth 9 Will be updated soon
Debris 8
Rock 5

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 7 Will be updated soon
Shallow (0.5 to 3 m) 7
Medium (3 to 8 m) 8
Deep (8 to 15 m) 7
Very deep (> 15 m) 6

Rate of movement

Descriptor Rating Notes
Moderate to fast 3 Will be updated soon
Slow 8
Very slow 9
Extremely slow 9

Ground water conditions

Descriptor Rating Notes
Artesian 4 Will be updated soon
High 5
Low 8
Absent 8

Surface water

Descriptor Rating Notes
Rain 6 Will be updated soon
Snowmelt 6
Localized 3
Stream 3
Torrent 0
River 0

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 6 Will be updated soon
Feasibility and Manageability 8 Will be updated soon

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 8 Will be updated soon
Environmental suitability 6 will be updated
Economic suitability (cost) 8 Will be updated soon

References

  • Alonso E.E, Gens A., Lloret A. (1993). “The landslide of Cortes de Pallas, Spain”. Geotechnique, Vol. 43 (4), 507-521

  • Baker R.F., Marshall H.C., (1958). “Control and correction”. In: Special Report 29: Landslides and Engineering Practice, E.B. Eckel (eds.), HRB, National Research Council, Washington D.C., 150-188.

  • Chatwin S.C., Hiwes D.E., Schwab J.W., Swanston D.N. (1994). “A guide for management of landslide-prone terrain in the Pacific Northwest”. Second Edition, Ministry of Forest. http://www.for.gov.bc.ca/hfd/pubs/docs/Lmh/Lmh18.htm .

  • Peck R.B., Ireland H.O. (1953). “Investigation of stability problems”. Proc. American railway Engineering Association. Vol. 54, 1116-1128.

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